Method for separating a natural gas stream into a methane-enriched fraction and a fraction enriched in c2 and higher hydrocarbons

ABSTRACT

A process for purifying a feed gas including methane and heavy hydrocarbons, including: step a): cooling the feed gas in a heat exchanger; step b): introducing the resulting into a first phase separator to produce a liquid stream depleted in methane and enriched in heavy hydrocarbons and a gas stream; step c): separating the gas stream in a membrane from which a methane-enriched permeate stream and a partially condensed residue stream exit; step d): introducing the residue stream from step c) into a second phase separator vessel in order to produce a liquid stream and a gas stream; step e): introducing at least one portion of the gas stream resulting from step d) into a JT expansion means; and step f): heating at least one portion of the expanded stream in the heat exchanger used in step a) counter-current to the feed stream in order to cool the latter.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a 371 of International PCT ApplicationPCT/FR2018/051876, filed Jul. 23, 2018, which claims priority to FrenchPatent Application No. 1757155, filed Jul. 27, 2017, the entire contentsof which are incorporated herein by reference.

BACKGROUND

The present invention relates to a process for purifying a gascontaining hydrocarbons heavier than methane, for example natural gas ora gas associated with oil production or a flare gas or a gaseouseffluent from a refinery. Most conventional units used for extractingnatural gas liquids (NGLs) or liquid petroleum gases (LPGs) arecryogenic units. These are generally expensive and consume a lot ofelectricity. Some alternative membrane technologies make it possible toconcentrate the natural gas liquids on the residue (retentate) side of amembrane. The Applicant has, for example, developed a polymer fiber,resistant to the formation of liquids and selective with respect tomethane compared to hydrocarbons containing more than two or more thanthree carbon atoms: light hydrocarbons (and hydrogen) permeate while thepartial pressure of heavier hydrocarbons increases on the high pressureside (residue side of the membrane) thus resulting in partial or evencomplete condensation of the heavy hydrocarbons.

SUMMARY

The inventors of the present invention have developed a solutionenabling the separation of a gas stream into a methane-enriched fractionand a fraction enriched in C2 and higher hydrocarbons, minimizing thelosses of methane during this removal and while minimizing the costsinvolved in the deployment of processes of this type.

The subject of the present invention is a process for purifying a feedgas stream comprising methane, and hydrocarbons containing at least twocarbon atoms, comprising the following steps:

-   -   Step a): cooling the feed gas stream in a heat exchanger;    -   Step b): introducing the stream resulting from step a) into a        first phase separator vessel in order to produce a liquid stream        depleted in methane and enriched in hydrocarbons containing more        than two carbon atoms and a gas stream;    -   Step c): separating the gas stream resulting from step b) in a        membrane permeation unit from which at least one        methane-enriched gaseous permeate stream and one partially        condensed residue stream enriched in hydrocarbons containing at        least two carbon atoms exit;    -   Step d): introducing the residue stream resulting from step c)        into a second phase separator vessel in order to produce at        least two phases including a liquid stream and a gas stream;    -   Step e): introducing at least one portion of the gas stream        resulting from step d) into a Joule-Thomson expansion means;    -   Step f): heating at least one portion of the expanded stream        resulting from step e) by introduction into the heat exchanger        used in step a) counter-current to the feed stream in order to        cool the latter.

According to other embodiments, a subject of the invention is also:

-   -   A process as defined above, characterized in that the heated        stream resulting from step f) is recycled by mixing with the        feed stream.    -   A process as defined above, characterized in that at least one        portion of the liquid stream resulting from step d) is        introduced into the heat exchanger used in step a)        counter-current to the feed stream,    -   A process as defined above, characterized in that at least one        portion of the liquid stream resulting from step b) is mixed        with said at least one portion of the liquid stream resulting        from step d) before being introduced into the heat exchanger        used in step a) counter-current to the feed stream.    -   A process as defined above, characterized in that at least one        portion of the methane-enriched permeate stream resulting from        step c) is heated by introduction into the heat exchanger used        in step a) counter-current to the feed stream in order to cool        the latter.    -   A process as defined above, characterized in that said at least        one portion of the permeate stream resulting from step d)        undergoes a Joule-Thomson expansion prior to the introduction        thereof into the heat exchanger,    -   A process as defined above, characterized in that at least one        portion of the liquid stream resulting from step d) is        introduced into a third phase separator vessel in order to        produce at least two phases, including a liquid stream and a gas        stream.    -   A process as defined above, characterized in that said gas        stream at the outlet of the third phase separator vessel is        mixed is heated by introduction into the heat exchanger used in        step a) counter feed stream in order to cool the latter. The        expression “feed stream” as used in the present patent        application relates to any composition containing hydrocarbons,        including at least methane.

The heat exchanger may be any heat exchanger, any unit or otherarrangement suitable for allowing the passage of a certain number ofstreams, and thus allowing direct or indirect exchange of heat betweenone or more refrigerant fluid lines and one or more feed streams.

Preferably, the purified stream (liquid fraction of the residue)comprises at least 50 mol % of hydrocarbons other than methane.

The membrane separation unit used during step c) has a greaterselectivity for methane than for hydrocarbons containing at least twocarbon atoms, preferably containing at least three carbon atoms andoperates in the presence of liquid. Methane or even hydrogen is found inthe permeate stream at the outlet of the membrane unit while thehydrocarbons heavier than methane are on the residue (retentate) side,giving rise to a partial or complete condensation of the liquid residuestream rich in hydrocarbons comprising at least two carbon atoms.

The present invention consists of the combination of a membrane unitwith partial or complete condensation on the residue (retentate) sideand a heat exchanger between the feed gas cooled by the expanded gasfraction or the expanded liquid fraction of said residue in order toeffectively separate methane from the heavier hydrocarbons. The lowertemperature thus obtained for the feed stream makes it possible toincrease the rate of formation of liquid hydrocarbons.

BRIEF DESCRIPTION OF THE DRAWING

An example of use of the present invention is illustrated in the figureby the following example.

In the figure, a natural gas feed stream 1 is introduced into a heatexchanger 2 at a temperature T1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Typically the feed stream 1 may contain methane, ethane, propane,hydrocarbons containing at least four carbon atoms, CO₂, aromatics,nitrogen, water, sulfur compounds (H₂S for example).

The feed stream 1 may be compressed via a compressor 24 for example, sothat the pressure is sufficient for the correct implementation of theprocess according to the invention.

A partially condensed stream 3 leaves the heat exchanger 2 at atemperature T2 below T1.

The stream 3 is introduced into a phase separator vessel 4 from which awater-rich liquid stream 5′, a liquid stream 5 rich in hydrocarbonscontaining at least two carbon atoms and a methane-enriched gas stream 6emerge, The stream 3 may have been expanded to a stream 3′, via a JouleThomson valve 26 for example, before it enters the separator vessel 4.

The gas stream 6 is then introduced into a membrane separation unit 7having a greater selectivity for methane than for hydrocarbonscontaining more than two carbon atoms and operating in the presence offormation of liquid on the residue (retentate) side. In this membraneunit 7, the stream is separated into a methane-enriched gaseous permeatestream 8, the pressure of which is lower than the pressure of the stream6, and a partially condensed residue stream 9 enriched in at least onehydrocarbon containing more than two carbon atoms.

The stream 9 is introduced into a phase separator vessel 10. A liquidstream 11 rich in hydrocarbons containing at least two carbon atoms anda gas stream 12 emerge therefrom.

The gas stream 12 can then, at least in part, be heated 15 in the heatexchanger 2 to a temperature fairly close to T1 (i.e. to a temperatureT4 strictly above T2 and at least between T2 and T1). The stream 12 isheated in the heat exchanger 2 in the counter direction to the feedstream 1 which, in itself, is cooled to the temperature T2.

The stream 15 can then optionally be mixed with the feed stream 1 inorder to be recycled. This recycling may take place after passingthrough a dedicated compressor 25 depending on the requirements andoperating conditions if a treated gas compressor 24 is not used.

Prior to entering the heat exchanger 2, the stream 12 is expanded 14using a Joule-Thomson valve 13.

It is also possible to introduce the liquid streams 5 and 11, at leastin part, independently or after having been mixed 17 and optionallyexpanded via a Joule Thomson valve 36, into the heat exchanger 2 inorder to be heated 27 and partially (or even completely) vaporized andused to cool the feed stream 1.

A portion of the liquid stream 11 can be introduced into a phaseseparator vessel 21 after having been expanded 20 via a Joule Thomsonvalve 28. From this phase separator vessel 21, a liquid phase 22enriched in heavy hydrocarbons and a gas phase 23 emerge.

The gas phase 23 and/or at least one portion 18 of the methane-enrichedpermeate stream 8 exiting the membrane unit 7 can be introduced(optionally after mixing), after an optional expansion 19, into the heatexchanger 2 in order to serve as a cold source for cooling the feed gas1. This results in a stream 29 which will be able to serve, at least inpart, as methane-enriched fuel 30 if need be. At least one other portion31 of this stream 29 could be mixed again with the portion 32 of thepermeate stream 8 which has not been sent to the heat exchanger 2.

It is also possible for a portion 33 of the gas stream 12 leaving thephase separator vessel 10 to be drawn off in order to be mixed with thestream 32 or with the stream 30.

This mixing may take place after expansions via Joule Thomson valves 34and 35 for example.

It will be understood that many additional changes in the details,materials, steps and arrangement of parts, which have been hereindescribed in order to explain the nature of the invention, may be madeby those skilled in the art within the principle and scope of theinvention as expressed in the appended claims. Thus, the presentinvention is not intended to be limited to the specific embodiments inthe examples given above.

1.-9. (canceled)
 10. A process for purifying a feed gas streamcomprising methane and hydrocarbons containing at least two carbonatoms, comprising the following steps: Step a): cooling the feed gasstream in a heat exchanger; Step b): introducing the stream resultingfrom step a) into a first phase separator vessel in order to produce aliquid stream depleted in methane and enriched in hydrocarbonscontaining more than two carbon atoms and a gas stream; Step c):separating the gas stream resulting from step b) in a membranepermeation unit from which at least one methane-enriched gaseouspermeate stream and one partially condensed residue stream enriched inhydrocarbons containing at least two carbon atoms exit; Step d):introducing the residue stream resulting from step c) into a secondphase separator vessel in order to produce at least two phases includinga liquid stream and a gas stream; Step e): introducing at least oneportion of the gas stream resulting from step d) into a Joule-Thomsonexpansion means; Step f): heating at least one portion of the expandedstream resulting from step e) by introduction into the heat exchangerused in step a) counter-current to the feed stream in order to cool thelatter.
 11. The process as claimed in claim 10, wherein the heatedstream resulting from step f) is recycled by mixing with the feedstream.
 12. The process as claimed in claim 10, wherein at least oneportion of the liquid stream resulting from step d) is introduced intothe heat exchanger used in step a) counter-current to the feed stream.13. The process as claimed in claim 12, wherein at least one portion ofthe liquid stream resulting from step b) is mixed with said at least oneportion of the liquid stream resulting from step d) before beingintroduced into the heat exchanger used in step a) counter-current tothe feed stream.
 14. The process as claimed in claim 10, wherein atleast one portion of the methane-enriched permeate stream resulting fromstep c) is heated by introduction into the heat exchanger used in stepa) counter-current to the feed stream in order to cool the latter. 15.The process as claimed in claim 14, wherein said at least one portion ofthe permeate stream resulting from step d) undergoes a Joule-Thomsonexpansion prior to the introduction thereof into the heat exchanger. 16.The process as claimed in claim 10, wherein at least one portion of theliquid stream resulting from step d) is introduced into a third phaseseparator vessel in order to produce at least two phases, including aliquid stream and a gas stream.
 17. The process as claimed in claim 16,wherein said gas stream at the outlet of the third phase separatorvessel is mixed with the permeate stream resulting from step c).
 18. Theprocess as claimed in claim 16, wherein the gas stream at the outlet ofthe third phase separator vessel is heated by introduction into the heatexchanger used in step a) counter-current to the feed stream in order tocool the latter.